We know that an analogous decomposition of salicin is produced by emulsin; neither in the one case nor in the other, is it possible to detect a physiological act. But between 1855 and 1875 Pasteur established unequivocally (i) the role of yeast in alcoholic fermentation, (ii) fermentation as a physiological phenomenon, and (iii) differences between the aerobic and anaerobic utilization of sugar by yeasts. The first part of Pasteur's paper deals with the changes in sugar which are brought about by alcoholic fermentation. The second part considers especially the "ferment", its nature, and the transformations it undergo. Pasteur's work on beer and wine yeasts gives some account of different yeasts, although he was never much interested in taxonomy. Indeed, his work Études sur la Bière described some elegant experiments on yeasts associated with wine grapes, probably carried out in the autumn of 1872. In 1862 Pasteur had discussed the sources of wine yeasts, describing how yeasts could be found in different fruit juices of high acidity, although if the juices were less acidic, bacteria would grow too. By 1880, alcoholic fermentation as a sign of the physiological activity of yeasts was not quite yet scientific orthodoxy. Up to that time, the finding of independent enzymic activity, separated from that of living cells, impeded understanding of the role of enzymes in cellular activity.

Pasteur’s apparatus for sterilizing and removing oxygen from a sugar solution in flask A. After the flask was cooled, the end of the curved tube was placed under mercury, as in Fig. 2.4 (1704, Fig. 60).

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FIGURE 2.3

Pasteur’s apparatus for sterilizing and removing oxygen from a sugar solution in flask A. After the flask was cooled, the end of the curved tube was placed under mercury, as in Fig. 2.4 (1704, Fig. 60).

Glass bulb used by Pasteur to examine yeast cells under a microscope. A tube is blown out into a flat bulb, the sides of which in the center are sufficiently close together to contain only a thin layer of liquid (1704, Fig. 31).

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FIGURE 2.6

Glass bulb used by Pasteur to examine yeast cells under a microscope. A tube is blown out into a flat bulb, the sides of which in the center are sufficiently close together to contain only a thin layer of liquid (1704, Fig. 31).

Pasteur’s drawings of the cells of two kinds of yeast found in fermenting grape must: he called the small lemon-shaped yeast Saccharomyces apiculatus and the larger round-celled yeast Saccharomyces pastorianus or Saccharomyces ellipsoideus (1704, Fig. 27).

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FIGURE 2.8

Pasteur’s drawings of the cells of two kinds of yeast found in fermenting grape must: he called the small lemon-shaped yeast Saccharomyces apiculatus and the larger round-celled yeast Saccharomyces pastorianus or Saccharomyces ellipsoideus (1704, Fig. 27).